1. Field of the Invention
The present invention relates to an optical recording medium suited for optical writing recording by use of a laser beam, particularly a semiconductor laser beam, and, more particularly, to an improved optical recording medium that can be used in optical disk techniques and optical card techniques.
2. Related Background Art
In general, optical recording mediums as exemplified by optical disks and optical cards can record information in a high density by forming optically detectable pits of minute size, for example, of about 1 .mu.m on a thin recording layer provided on a substrate having grooves of spiral, circular or linear form.
A focused laser beam may be scanned on the surface of this recording layer, so that the recording layer can absorb laser beam energy to form optically detectable pits thereon, and thus the writing of information can be performed. For example, in a heat mode recording system, the recording layer absorbs a heat energy to form minute concaves, i.e., pits, on that energy-absorbed parts by evaporation or fusion. In another heat mode system, the absorption of the energy of the irradiated laser beam can form pits having an optically detectable density difference on those parts.
The information recorded on the optical disks or optical cards can be detected by reading optical changes between the part on which the pits are formed and the part on which the pits are not formed. For example, a laser beam is scanned along a track, and the energy reflected by a disk is monitored by a photodetector. At the part on which the pits are formed, the reflection of the laser beam is lowered and the output from the photodetector becomes smaller. On the other hand, at the part on which the pits are not formed, the laser beam is sufficiently reflected and the output form the photodetector becomes larger.
Hitherto proposed as the optical recording mediums used in such optical disks and optical cards are those in which inorganic materials are mainly used, for example, metallic thin films such as aluminum-deposited films, bismuth thin films, tellurium oxide thin films, chalcogenite type amorphous glass films or the like. These thin films, however, have disadvantages such that they are sensitive to light of wavelengths generally of from 350 to 800 nm and also have a high reflectance to laser beams, resulting in a low utilization of laser beams.
Taking account of the above, researches have been made on organic thin films capable of optically causing changes in physical properties by using optical energy of relatively long wavelengths (for example, 780 nm or more). Such organic thin films are effective as being capable of forming pits with use of a semiconductor laser beam having an oscillation wavelength near 780 nm or 830 nm.
However, organic compounds that commonly have absorption characteristics on the longer wavelength side have a problem that they are unstable to heat.
For example, in handling optical recording mediums, they are not necessarily used only in well air-conditioned offices, and there must be taken into account their transportation, keeping in ware houses and temperature rise inside driving units, so that mediums having far superior stability to high-temperature have been sought after. In particular, potable optical recording mediums as exemplified by optical cards may preferably have the laminated structure that an opposed substrate is directly laminated on the recording layer with use of an adhesive, from the viewpoint of thickness or strength of the mediums.
Effectively used as the adhesive used in that occasion are hot-melt adhesives which have good workability. Since, however, many of the hot-melt adhesives have temperatures of about 100.degree. C. at the time of lamination, sought after as recording layer is the one having far superior thermal resistance. Since also the optical cards are often handled by being personally carried, thus giving a high possibility that they are roughly handled, those having superior environmental stability including the thermal resistance have been sought after.
As a coloring matter that can solve such problems, U.S. Pat. No. 4,548,886 discloses an azulene coloring matter. More specifically, this coloring matter has superior optical characteristics, i.e., clear threshold characteristics of recording sensitivity and high recording sensitivity, and also can be formed into a film by coating.